1. Field of the Invention
The present invention relates to a block transformation coding system which is used in transmission of image data. The present invention relates to, in particular, a block transformation coding system which is used in transmission of image data having a wide band width.
2. Description of the Related Art
In transmission of image data, a block transformation coding system such as an orthogonal transformation coding system, is used to reduce the bit rate.
For example, in the orthogonal transformation coding system, a field of image data is divided into a plurality of blocks, image data in each block is transformed into frequency components, the transformed data is quantized in a quantizer, and is transmitted on a transmission line.
The bit rate after the block transformation before the quantizing, varies depending on the degrees of motion in the picture or variation of the image in each field. As the transmission rate in the transmission line has an upper limit, the bit rate transmitted on a transmission line is adaptively controlled based on the total amount of data which is to be transmitted, by changing a mode of the quantization in the quantizer.
In the above control of the quantization mode, a reduction of higher frequency components is often carried out. However, this reduction causes a deformation in a decoded image in a receiver side, e.g., a prominent blocklike pattern appears in the decoded image in the receiver side.
On the other hand, coding image data having a wide band width such as image data in a High Definition Television (HDTV) system, which requires a high processing speed, cannot be processed by one coder. Therefore, image data in a field is delivered to and developed in two groups, and the groups of image data are respectively coded in a plurality of coder, in parallel processing.
FIG. 1 shows an outline of FIGS. 1A and 1B illustrating an example of the above delivery to two groups and development therein of image data. In FIGS. 1A and 1B, L is a number of pixels in each line of the original image data. In the example of FIGS. 1A and 1B an original image is divided into a plurality of blocks each consisting of 8.times.4 pixels as shown in FIG. 1A, where pixels in each even-numbered line are divided into portions (8m+1)-th to 8(m+1)-th pixels (m=0-n, and 8n is equal to the number L of the total pixels in each line), and pixels in each odd-numbered line are divided into portions (8m+2)-th to [8(m+1)+1]-th pixels. Each odd-numbered pixel in each even-numbered line and each even-numbered pixel in each odd-numbered line are developed in a field of the group 1, and each even-numbered pixel in each odd-numbered line and each odd-numbered pixel in each even-numbered line are developed in a field of the group 2.
The image data in the groups 1 and 2 is respectively coded in a plurality of block transformation coders and transmitted to a receiver side. A decoder in the receiver side decodes the transmitted data in the groups 1 and 2 , respectively, and composes the decoded image data in the groups 1 and 2 to form the original image data in a manner inverse from FIGS. 1A and 1B.
However, in the conventional coding system wherein an original image data is developed on two groups, and is coded by a plurality of coders, as explained above, the dividing lines (boundaries) of the blocks are the same for the data in both the groups 1 and 2. Therefore, the aforementioned blocklike pattern is emphasized in the decoded image by the above parallel coding using two coders.